In recent years, thermal transfer systems have been developed to obtain prints from pictures that have been generated from a camera or scanning device. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye receiver element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to one of the cyan, magenta or yellow signals. The process is then repeated for the other colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen.
Dye receiver elements used in thermal dye transfer generally include a support (transparent or reflective) bearing on one side thereof a dye image-receiving layer, and optionally additional layers, such as a compliant or cushioning layer between the support and the dye receiving layer. The compliant layer provides insulation to keep heat generated by the thermal head at the surface of the print, and also provides close contact between the donor ribbon and receiving sheet which is essential for uniform print quality.
Various approaches have been suggested for providing such a compliant layer. U.S. Pat. No. 5,244,861 (Campbell et al.) describes a composite film comprising a microvoided core layer and at least one substantially void-free thermoplastic skin layer. Such an approach adds an additional manufacturing step of laminating the previously created composite film to the support, and film uniformity can be variable resulting in high waste factors. U.S. Pat. No. 6,372,689 (Kuga et al.) describes the use of a hollow particle layer between the support and dye receiving layer. Such hollow particles layers are frequently coated from aqueous solutions that necessitate a powerful drying stage in the manufacturing process and may reduce productivity. In addition, the hollow particles with varied size and size distribution may result in increased surface roughness in the finished print that reduces surface gloss. It would be advantageous to provide a compliant layer that enables a high gloss print to be obtained. It would also be advantageous if the technology used to provide such a compliant layer also enables a matte print to be obtained if a low gloss finish is desired. It would also be advantageous if the technology used enables any intermediate finishes between glossy and matte finishes.
U.S. Pat. No. 6,897,183 (Arrington et al.) describes a process for making a multilayer film, useful in an image recording element, wherein the multilayer film comprises a support and an outer or surface layer and between the support and the outer layer is an “antistatic tie layer” comprising a thermoplastic antistatic polymer or composition having preselected antistatic properties, adhesive properties, and viscoelastic properties. Such a multilayer film may be used in making a thermal-dye-transfer receiver element comprising a support and a dye receiving layer wherein between the support and the dye receiving layer is a tie layer. However, this patent fails to mention the importance of tie layer adhesion to the dye receiving layer and to the support during printing and immediately after the print is made. Also, no mention is made of the importance of printing under hot and humid conditions, and lack of humidity sensitivity of the tie layer compositions. U.S. Patent Application Publication 2004/0167020 (Arrington et al.) has similar disclosure in that it does not make any reference to adhesion of the dye receiver layer to the support during printing, immediately after printing, printing under hot and humid conditions, or humidity sensitivity of tie layer compositions.
Known polymer compliant composite laminates used on the faceside (imaging side) of dye-thermal receiver elements generally have a top skin layer of polypropylene (PP) onto which can be extruded a dye receiver layer (DRL) containing a polyester/polycarbonate blend. A known tie layer used between the composite laminate support and the dye receiving layer (DRL) is antistatic and is a blend of 70 wt. % PELESTAT® 300 (polyethylene-polyether copolymer) and 30 wt. % polypropylene (PP). The rheology of these two components is such that PELESTAT® 300 encapsulates the polypropylene (PP), so that the continuous phase in the tie layer is PELESTAT® 300. The PELESTAT® 300 acts as an antistatic material as well as an adhesive component to polymer laminate support skin layer and the dye receiving layer (DRL). This tie layer, however, is significantly humidity sensitive, has poor adhesion, and does not survive borderless printing (edge to edge) when tested under hot and humid conditions such as 36° C./86% RH. Moreover, as stated previously, the application of a composite laminate film requires an additional manufacturing step.
There remains a need to provide a compliant layer using technology that is highly efficient from a manufacturing viewpoint and has enhanced adhesion between supports or substrates, tie layers, and receiving layers extruded onto the substrates or supports to avoid delamination during printing, especially when adhesion is negatively affected by humidity. It would also be desirable for the image-receiving layer to be readily applied to the underlying support with adequate adhesion. It further would be desirable for the compliant layer and tie layers to be co-extrudable to reduce the number of manufacturing operations, or even to co-extrude the compliant layer, antistatic tie layer, and dye receiving layers for most efficient manufacture. It is further desirable that the extruded layer technology would allow for either a glossy or matte print to be obtained.